1. Field of the Invention
The present invention relates to a new and improved modulating pilot valve for adjusting product pressure relief flow from a balanced main valve.
2. Description of the Background Art
It is desirable to secure a safety or pressure relief valve on a pressure vessel for venting product pressure that exceeds a limit above which a dangerous condition may result. A typical safety valve includes a valve member biased by a spring into engagement with a seat. If product pressure exceeds a predetermined safe level, the force against the valve exceeds the biasing force of the spring causing the valve to lift from the seat to vent product pressure. Such valves are unreliable and subject to rapid deterioration due to repeated engagement with the valve seat. Since the valve and seat are in the flow of product fluid, damage from the impinging flow also occurs. Safety valves incorporating a pilot valve have also been employed. Also valves now in use operate by continuously bleeding the product from the main valve resulting in substantial loss of product. In addition, continuous pilot fluid flow or bleed through the pilot valve increases the probability that debris entrained in the fluid may obstruct the pilot valve or freezing can occur.
A known safety valve for a pressure vessel is illustrated in U.S. Pat. No. 4,291,721. The valve disclosed in this patent does not provide modulating action, and the extended construction utilized increases the possibility of malfunction by sticking of close fitting sliding parts.
All of the existing valves as described are difficult to test, i.e. to make a field determination of pilot valve operating pressure when installed on a pressure vessel or conduit. With these valves, testing generally requires venting of a substantial volume of product fluid.